Generally, secondary batteries capable of repeated charging and discharging are widely used as power sources for electronic devices, including mobile phones, notebook computers and camcorders, and electric automobiles, unlike primary batteries incapable of charging and discharging. Particularly, lithium secondary batteries are operated at a voltage of 3.6 V, have a capacity about three times higher than nickel-cadmium batteries or nickel-metal hydride batteries that are mainly used as power sources for electronic systems, and have a high energy density per unit weight. Due to these advantages, the practical use of lithium secondary batteries is growing rapidly.
In such lithium secondary batteries, lithium oxides and carbon materials are used as cathode active materials and anode active materials, respectively. Lithium secondary batteries can be classified into prismatic, cylindrical and pouch-type batteries.
A typical lithium secondary battery includes an electrode assembly and an exterior material adapted to sealably house the electrode assembly together with an electrolyte solution therein. The electrode assembly includes a cathode, a separator and an anode arranged sequentially. Particularly, the exterior material of the secondary battery is prismatic or cylindrical in structure and includes a case formed with an open end and a cap assembly sealably coupled to the open end of the case.
Electrode assemblies are classified into jelly-roll type electrode assemblies and stack type electrode assemblies. A jelly-roll type electrode assembly is constructed by interposing a separator between a cathode and an anode, and winding the electrode structure. A stack type electrode assembly is constructed by alternately laminating a plurality of cathodes and anodes, each of which has a predetermined size, between which separators are interposed. The jelly-roll type electrode assembly is easy to construct and has an advantage of high energy density per unit weight. Particularly, the jelly-roll type electrode assembly is easily housed in a case for a cylindrical or prismatic battery. Due to these advantages, the jelly-roll type electrode assembly is widely used for battery fabrication. On the other hand, the stack type electrode assembly is widely used in pouch-type secondary batteries.
FIG. 1 is a cross-sectional view of a conventional cylindrical secondary battery.
Referring to FIG. 1, the cylindrical secondary battery 10 includes a cylindrical can 20, a jelly-roll type electrode assembly 30 accommodated in the can 20, a cap assembly 40 coupled to the top of the can 20, a beading portion 21 formed by inwardly bending the wall of the can 20 to mount the cap assembly 40 thereon, and a crimping portion 50 adapted to seal the battery.
The electrode assembly 30 has a structure in which a cathode 31, an anode 32 and a separator 33 interposed between the two electrodes are wound in a jelly-roll configuration. A cathode tab 34 attached to the cathode 31 is connected to the cap assembly 40, and an anode tap (not shown) attached to the anode 32 is connected to the bottom of the can 20.
The cap assembly 40 has a structure in which a top cap 41, a positive temperature coefficient (PTC) element 42, a safety vent 43, an insulating member 44 and a cap plate 45 are laminated in this order from the top. The top cap 41 forms a cathode terminal, the PTC element 42 interrupts an electric current when the resistance of the battery increases with increasing internal temperature of the battery, the safety vent interrupts an electric current and/or exhausts gases when the internal pressure of the battery increases, the insulating member 44 electrically disconnects the safety vent 43 from the cap plate 45 except a predetermined portion, and the cap plate 45 is connected to the cathode tap 34 connected to the cathode 31. The cap assembly 40 is mounted in a gasket 60 and is mounted on the beading portion 21.
The cylindrical secondary battery is generally fabricated by inserting the jelly-roll type electrode assembly into the cylindrical can, forming the beading portion at a position of the can corresponding to the outer circumference of the top end of the electrode assembly, mounting the cap assembly provided with a packing on the beading portion, followed by subjecting the top end of the can to crimping and sizing processes. According to the sizing process, the battery is pressed using a mold in a state in which an injection hole is closed. As a result, the width of the beading portion is reduced, ensuring an internal space of the battery.
The sizing process for reducing the width of the beading portion causes downward deformation of the inner side portion of the beading portion. This downward deformation may bring about a reduction in the internal space of the battery, leading to a low capacity of the battery compared to a battery with the same specification. Further, the downwardly deformed beading portion may be brought into contact with the top end of the electrode assembly or the electrode tab. In this case, there is a possibility of short circuiting. Accordingly, the structure of the beading portion and the sizing process may cause serious problems in terms of battery safety. On the other hand, the mold is rapidly moved downward during the size process to prevent the beading portion from returning to the original width. This strong impact from the mold may increase the risk of damage to the electrode assembly.